1. Field of the Invention
The present invention relates to a high-frequency heat-sealing apparatus for use in a filling/packing machine for producing liquid beverages or the like which are packed in paper containers so that they can be stored for a long time and, more particularly, to a sealing apparatus for transversely heat-sealing a tubular packing material filled with fluid contents, such as liquid beverages, in the presence of the fluid contents.
2. Description of the Related Art
Hitherto, when a packing container having a box shape filled with a liquid such as juice in a sealed manner is to be produced, as seen in Japanese Patent Publication No. 3215/1980, it is well known to use a sealing apparatus in which a packing material of a laminate of paper, an aluminum foil and a synthetic resin is used and continuously formed into a tubular shape while being conveyed. The tubular packing material is transversely heat-sealed in the presence of the fluid, by using pressing means including a sealing jaw having a high-frequency coil and an opposing jaw having a sealing rubber made of a hard rubber, and is cut between two sealed zones, having been heat-sealed, by a cutting blade.
In Japanese Patent Laid-Open Patent Application Nos. 134744/1983, 269854/1993, 164523/1995 and 240607/1997, there is further disclosed a heat-sealing apparatus for heat-sealing a laminate material in which a ridge having a rectangular shape in the cross section is formed on the action face of a high-frequency coil on a sealing jaw in the aforementioned heat-sealing apparatus so that the molten resin is extruded into a sealed zone by the ridge.
In the aforementioned heat-sealing apparatus, when the molten resin is guided to flow toward the container's interior side of the sealed portion of the tubular packing material, corrugated molten resin beads are formed on the edge portion of the container's interior side, so that cracks start from the crests of the beads to break the container when an external force is applied to the container. In order to avoid this problem, two magnetic members are arranged on the outer side (or on the container's interior side) of the straight portions of a U-shaped high-frequency coil to leave the portion of the container inner side unheated, as disclosed in Japanese Patent No. 2,571,977.
In Japanese Patent Laid-Open Patent Application No. 230834/1996, it is described that in the aforementioned heat-sealing apparatus a high-frequency coil, which has a ridge in its longitudinal direction and grooves formed on the two sides of and in parallel with the ridge, molten resin is caused to remain in the grooves without flowing out of the sealed zone, even if the resin on the packing material is melted and pushed by the ridge.
In Japanese Patent Laid-Open Patent Application No. 244728/1996, a high-frequency coil is used which is inclined gradually as the sealed zone forming portion approaches the outer side (the container's interior side). The tube is thus heat-sealed whereby the liquid and the molten resin are discharged smoothly from the sealed zone to the container's interior side, when the tube is pressed together in the presence of a fluid.
In Japanese Patent Laid-Open Patent Application No. 52025/1987, it is disclosed that when an electric current flows in a high-frequency coil, a high-frequency electromagnetic field is generated around the high-frequency coil and induces an eddy current in a conductive material such as an aluminum foil, so that heat is generated in the conductive material by the eddy current to heat and melt a thermoplastic resin adjacent to the conductive material. However, the eddy current thus induced establishes a rounded electric circuit at each of the two end portions of the conductive material such as an aluminum foil, so that the heated regions at the two end portions are also rounded, thus narrowing the width of the thermoplastic material layer to be melted at the two end portions. There is also disclosed a high-frequency coil which has a linear ridge for pushing the center parts of the sealed zone,at the two narrowed right and left side portions or at the non-side portion of the sealed zone.
In order to effect the heat-sealing properly in a high-frequency heat-sealing apparatus for transversely heat-sealing a tubular packing material filled with contents such as a liquid beverage in the presence of the liquid, it is necessary to push the thermoplastic material layers melted by the high-frequency heating, with a ridge provided on the high-frequency coil and to extrude the molten thermoplastic resin together with contaminants such as liquid beverages existing between the thermoplastic material layers, from under the ridge to the two sides of the same, thereby forming a thin layer of the thermoplastic resin containing no contaminant below the ridge.
Here, if the tubular packing material having an aluminum foil is heated with the U-shaped high-frequency coil, as described in Japanese Patent Laid-Open No. 52025/1987, there is established a heating pattern forming a loop. As a result, the two right and left side portions of the sealed zone in the longitudinal direction are made narrower than at the non-side portion. For example, when the heat-sealing is effected by using a high-frequency heat-sealing apparatus which is equipped with a high-frequency coil having a linear ridge of a mismatched heating pattern such as a linear ridge for pushing the widthwise center of the non-side portion of the sealed zone, the ridge may push the non-molten region of the two right and left end portions of the tubular packing material, thus causing a problem that defective sealing may occur.